CONVERSION OF NOx IN EXHAUST GAS

ABSTRACT

A quantity of ammonia being consumed within an exhaust system of an internal combustion engine to reduce NOx in exhaust passing through the exhaust system by chemical reactions enabled by an SCR catalyst is calculated by processing, in accordance with an algorithm, certain data including data indicative of a quantity of NOx in exhaust upstream of where ammonia is being introduced into the exhaust system, data indicative of a quantity of NOx in exhaust downstream of the SCR catalyst, and data indicative of exhaust flow.

TECHNICAL FIELD

This disclosure relates generally to Selective Catalytic Reduction (SCR) of oxides of Nitrogen (NOx) in gaseous products of combustion, and more particularly to a system and method for estimating a quantity of ammonia (NH₃) consumed by SCR conversion of NOx in exhaust passing through an exhaust system of an internal combustion engine.

BACKGROUND

One engine exhaust technology for after-treatment of engine exhaust utilizes SCR to enable certain chemical reactions to occur between NOx and ammonia injected into an exhaust system. Those reactions convert NOx into Nitrogen (N₂) and water (H₂O), two constituents found in abundance in earth's atmosphere. NOx and ammonia are the only reactants in certain of those reactions while Oxygen (O₂), which may be present in the exhaust, is a third reactant in other reactions.

Ammonia is introduced in sufficient quantity to maintain a presence of ammonia on surfaces of the SCR catalyst where the chemical reactions to reduce NOx take place.

SUMMARY

A quantity of ammonia introduced into the exhaust system can be measured in various ways. However, that measurement doesn't necessarily equate to ammonia consumed to reduce NOx. If ammonia is being introduced in quantity which creates ammonia slip, some of the ammonia being introduced is not consumed and instead is exhausted into the atmosphere.

The presently disclosed subject matter relates to estimating a quantity of ammonia actually consumed in reducing NOx to N₂ and H₂O.

One general aspect of the disclosed subject matter relates to an internal combustion engine comprising combustion chambers within which fuel is combusted to operate the engine, an intake system through which air for supporting combustion is introduced into the combustion chambers, an exhaust system through which exhaust resulting from combustion in the combustion chambers passes to atmosphere and which comprises an SCR catalyst, and an ammonia delivery system for introducing ammonia into the exhaust system upstream of the SCR catalyst for entrainment with exhaust flow toward the SCR catalyst.

A processor processes certain data, including data indicative of a quantity of NOx in exhaust upstream of where ammonia is introduced into the exhaust system, data indicative of a quantity of NOx in exhaust downstream of the SCR catalyst, and data indicative of exhaust flow, in accordance with an algorithm to estimate a quantity of ammonia consumed by chemical reactions which are enabled by the SCR catalyst to reduce NOx into N₂ and H₂O.

Another general aspect of the disclosed subject matter relates to a method for estimating a quantity of ammonia consumed by chemical reactions which are enabled by an SCR catalyst within an exhaust system of an internal combustion engine to reduce NOx in exhaust passing through the exhaust system to N₂ and H₂O. The method comprises processing certain data, including data indicative of a quantity of NOx in exhaust upstream of where ammonia is being introduced into the exhaust system, data indicative of a quantity of NOx in exhaust downstream of the SCR catalyst, and data indicative of exhaust flow, in accordance with an algorithm to estimate a quantity of ammonia consumed by chemical reactions which are enabled by the SCR catalyst within the exhaust system.

Another general aspect of the disclosed subject matter relates to a system for estimating a quantity of ammonia consumed by selective catalytic reduction (SCR) for NOx reduction in exhaust flow through an exhaust after-treatment system. The system comprises an SCR catalyst, an ammonia supply, an ammonia outlet through which ammonia from the ammonia supply is introduced into the exhaust flow, a downstream NOx sensor providing data indicative of a quantity of NOx in exhaust flow downstream of the SCR catalyst, a data source providing data indicative of a quantity of NOx in exhaust flow upstream of the ammonia outlet, and a processor for processing data from the downstream NOx sensor, data from the data source, and data indicative of exhaust flow, in accordance with an algorithm to estimate a quantity of ammonia consumed to reduce NOx to N₂ and H₂O by chemical reactions enabled by the SCR catalyst.

The foregoing summary is accompanied by further detail of the disclosure presented in the Detailed Description below with reference to the following drawings which are part of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic diagram of an internal combustion engine which utilizes SCR to reduce NOx in engine exhaust by chemical reaction with ammonia introduced into the exhaust.

FIG. 2 is a diagram illustrating detail of a NOx reduction system present in FIG. 1.

FIG. 3 is a diagram of an algorithm for estimating a quantity of reductant (NH₃) consumed by chemical reaction with NOx in the NOx reduction system of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 shows a representative internal combustion engine 10 which can be used in stationary or mobile applications. For example, engine 10 may be a diesel engine which comprises structure forming a number of engine cylinders 12 into which fuel is injected by fuel injectors 14 to combust with air which has entered combustion chamber spaces of engine cylinders 12 through an intake system 16 when cylinder intake valves 18 for controlling admission of air from an intake manifold 20 into respective engine cylinders 12 are open. Other components which may be present in intake systems of contemporary diesel engines, such as a turbocharger compressor and charge air cooler, are not shown.

Engine 10 also comprises an exhaust system 22 through which engine exhaust created by combustion of injected fuel in the combustion chamber spaces to operate engine 10 is conveyed to atmosphere. Cylinder exhaust valves 24 control admission of exhaust from respective engine cylinders 12 into an exhaust manifold 26 for further conveyance through exhaust system 22.

Exhaust system 22 includes an exhaust after-treatment system 28 for treating exhaust prior to entry into the atmosphere. Other components which may be present in exhaust systems of contemporary diesel engines, such as a turbocharger turbine, are not shown.

A processor-based engine control module (ECM) 30 controls various aspects of engine operation, such as fueling of engine cylinders 12 by fuel injectors 14. Control is accomplished by processing various input data, indicated generally by reference numeral 32, to develop control data for control of functions being performed by various devices.

Exhaust after-treatment system 28 is shown in detail in FIG. 2 to comprise structure through which exhaust is constrained to pass. The particular structure shown comprises a generally cylindrical housing 34 having an axial length, an exhaust entrance 36 at an upstream axial end, and an exhaust exit 38 at a downstream axial end. Arrows 40 indicate a direction of exhaust flow into, through, and out of interior space of housing 34.

Within its interior space, housing 34 contains a diesel oxidation catalyst (DOC) 42 downstream of exhaust entrance 36 and an SCR catalyst 44 downstream of DOC 42. Exhaust which enters the interior space of housing 34 through exhaust entrance 36 is forced to pass first through DOC 42 and subsequently through SCR catalyst 44 before exiting through exhaust exit 38. DOC 42 treats engine exhaust by removing certain entrained matter, such as the soluble organic fraction of diesel particulate matter. SCR catalyst 44 treats engine exhaust by reducing NOx according to chemical reactions such as:

4NO+4NH3+O2→4N2+6H2O

NO+NO2+2NH3→2N2+3H2O

6NO2+8NH3→7N2+12H2O

SCR catalyst 44 may be a type which not only reduces NOx but also traps entrained particulate matter (soot) and therefore requires occasional regeneration.

An ammonia supply 46 stores ammonia which is used for NOx reduction Ammonia from supply 46 is introduced into the interior of housing 34 through a conduit 48 having an outlet 50 disposed at a location downstream of DOC 42 and upstream of SCR catalyst 44. FIG. 2 is intended to portray good distribution of ammonia within the exhaust flow without reference to specific structural details of how that is accomplished so that a continuing presence of ammonia can be assured over as large a surface area of SCR catalyst 44 as possible.

A processor-based ammonia dosing controller 52 controls the introduction of ammonia from ammonia supply 46 into housing 34 by processing various data, including data from ECM 30 with which it has communication. Controller 52 also processes data according to an algorithm for estimating a quantity of ammonia consumed by conversion of NOx in exhaust passing through exhaust system 22. That algorithm 54 will be explained with reference to FIG. 3.

The algorithm processes data indicating a quantity of NOx as measured at a location upstream of outlet 50 and data indicating a quantity of NOx as measured at a location downstream of SCR catalyst 44. These data are provided respectively by an upstream NOx sensor 56 upstream of DOC 42 and a downstream NOx sensor 58, both shown in FIG. 2. NOx sensor 58 is a type which also senses ammonia, and so the algorithm is premised on the assumption that ammonia is injected in quantity which causes minimal, nor no, slip. NOx sensor 56 may be replaced by a sufficiently accurate virtual NOx sensor. NOx sensor 56 may alternately be placed downstream of DOC 42 but upstream of ammonia outlet 50.

Controller 52 processes data from NOx sensor 56 indicating a quantity of NOx in untreated, i.e. “engine out”, exhaust and data from NOx sensor 58 indicating a quantity of NOx in exhaust which has been treated by SCR using ammonia stored on the surface of SCR 44. The processing performs a first calculation 60 which yields a fraction equal to the quantity of NOx indicated by downstream NOx sensor 58 divided by the quantity of NOx indicated by upstream NOx sensor 56. A second calculation 62 subtracts the calculated fraction from unity, leaving a resulting fraction representing the fraction of NOx which has been reduced.

Using the three chemical reactions given above and the result of calculation 62, algorithm 54 performs a calculation 64 of a quantity of ammonia which would have been consumed in order to reduce NOx to the resulting fraction calculated by step 62. Because the NOx sensors do not distinguish between NO and NO₂, the calculation may utilize a look-up table containing relative proportions of NO and NO₂ in the exhaust as a function of engine operation obtained by known techniques. The look-up table has been derived from previous mapping of engine exhaust at various engine operating conditions during engine development. Hence, algorithm 54 is repeatedly calculating the quantity of ammonia consumed to reduce NO and the quantity of ammonia consumed to reduce NO₂ over each interval of time between successive calculations as engine 10 operates. The calculation of the total quantity of ammonia consumed from an initial time to a present time is obtained by adding the respective quantities consumed to reduce NO and NO₂ during each interval between calculations. Because the calculation prior to a calculation 66 is based in parts per million (ppm), it must be converted from ppm to a flow rate [mass/time]. An estimate of the actual consumption is therefore calculated by calculation 66 which multiplies the result of calculation 64 by exhaust flow as measured or estimated in any suitably appropriate way. Depending on the unit of measurement of exhaust flow, a calculation 68 may be needed to convert the result of calculation 66 to desired units of ammonia consumption. 

What is claimed is:
 1. An internal combustion engine comprising: combustion chambers within which fuel is combusted to operate the engine; an intake system through which air for supporting combustion is introduced into the combustion chambers; an exhaust system through which exhaust resulting from combustion in the combustion chambers passes to atmosphere and which comprises an SCR catalyst; an ammonia delivery system for introducing ammonia into the exhaust system upstream of the SCR catalyst for entrainment with exhaust flow toward the SCR catalyst; and a processor for processing certain data including data indicative of a quantity of NOx in exhaust upstream of where ammonia is being introduced into the exhaust system, data indicative of a quantity of NOx in exhaust downstream of the SCR catalyst, and data indicative of exhaust flow, in accordance with an algorithm to estimate a quantity of ammonia consumed by chemical reactions which are enabled by the SCR catalyst to reduce NOx to N₂ and H₂O.
 2. The engine set forth in claim 1 including a downstream NOx sensor providing the data indicative of a quantity of NOx in exhaust downstream of the SCR catalyst.
 3. The engine set forth in claim 2 including an upstream NOx sensor providing the data indicative of a quantity of NOx in exhaust upstream of where ammonia is being introduced into the exhaust system.
 4. The engine set forth in claim 3 including a diesel oxidation catalyst in the exhaust system upstream of the SCR catalyst, and in which the upstream NOx sensor is disposed to indicate a quantity of NOx in the exhaust upstream of the diesel oxidation catalyst.
 5. The engine set forth in claim 1 in which the algorithm comprises a division calculation for dividing data indicative of a quantity of NOx in exhaust downstream of the SCR catalyst by data indicative of a quantity of NOx in exhaust upstream of where ammonia is being introduced into the exhaust system, a subtraction calculation for subtracting the division calculation from unity, and a multiplication calculation for multiplying the subtraction calculation a) by a quantity of ammonia which would have been consumed according to one or more chemical reactions in which at least ammonia and NOx are reactants in order to reduce NOx to the subtraction calculation, and b) by data indicative of exhaust flow through the SCR catalyst.
 6. The engine set forth in claim 1 in which the chemical reactions comprise one or more of: 4NO+4NH3+O2→4N2+6H2O NO+NO2+2NH3→2N2+3H2O 6NO2+8NH3→7N2+12H2O.
 7. The engine set forth in claim 5 in which the algorithm further comprises a conversion calculation for converting the multiplication calculation to desired units of ammonia consumption.
 8. A method for estimating a quantity of ammonia being consumed by chemical reactions which are enabled by an SCR catalyst within an exhaust system of an internal combustion engine to reduce NOx in exhaust passing through the exhaust system to N₂ and H₂O, the method comprising: processing certain data, including data indicative of a quantity of NOx in exhaust upstream of where ammonia is being introduced into the exhaust system, data indicative of a quantity of NOx in exhaust downstream of the SCR catalyst, and data indicative of exhaust flow, in accordance with an algorithm to estimate a quantity of ammonia consumed by chemical reactions enabled by the SCR catalyst within the exhaust system.
 9. The method set forth in claim 8 comprising using a downstream NOx sensor to provide the data indicative of a quantity of NOx in exhaust downstream of the SCR catalyst.
 10. The method set forth in claim 9 comprising using an upstream NOx sensor to provide the data indicative of a quantity of NOx in exhaust upstream of where ammonia is being introduced into the exhaust system.
 11. The method set forth in claim 9 comprising using an upstream NOx sensor to provide the data indicative of a quantity of NOx in exhaust upstream both of where ammonia is being introduced into the exhaust system and of a diesel oxidation catalyst which is upstream of the SCR catalyst in the exhaust system.
 12. The method set forth in claim 8 in which the processing comprises a division step of dividing data indicative of a quantity of NOx in exhaust downstream of the SCR catalyst by data indicative of a quantity of NOx in exhaust upstream of where ammonia is being introduced into the exhaust system, a subtraction step of subtracting a result of the division step from unity, and a multiplication step of multiplying a result of the subtraction step a) by a quantity of ammonia which would have been consumed according to chemical reactions in which at least ammonia and NOx are reactants in order to reduce NOx to the subtraction calculation and b) by data indicative of exhaust flow through the SCR catalyst.
 13. The method set forth in claim 12 further comprising converting a result of the multiplying step to desired units of ammonia consumption.
 14. A system for estimating a quantity of ammonia consumed by selective catalytic reduction (SCR) of NOx in exhaust flow through an exhaust system, the system comprising: an SCR catalyst; an ammonia supply; an ammonia outlet through which ammonia from the ammonia supply is introduced into the exhaust flow; a downstream NOx sensor providing data indicative of a quantity of NOx in exhaust flow downstream of the SCR catalyst; a data source providing data indicative of a quantity of NOx in exhaust flow upstream of where ammonia is being introduced into the exhaust system; and a processor for processing data from the downstream NOx sensor, data from the data source, and data indicative of exhaust flow, in accordance with an algorithm to estimate a quantity of ammonia consumed to reduce NOx to N₂ and H₂O by chemical reactions enabled by the SCR catalyst.
 15. The system set forth in claim 14 in which the algorithm comprises a division calculation for dividing data provided by the downstream NOx sensor by data provided by the data source, a subtraction calculation for subtracting the division calculation from unity, and a multiplication calculation for multiplying the subtraction calculation a) by a quantity of ammonia which would have been consumed according to one or more chemical reactions in which at least ammonia and NOx are reactants in order to reduce NOx to the subtraction calculation, and b) by data indicative of exhaust flow through the SCR catalyst.
 16. The system set forth in claim 14 in which the data source comprises an upstream NOx sensor. 